As for me, all my usernames are s(something). For the services that support it, the display name (or equivalent) is "sbird", because birds are awesome!
Most of my social related accounts, think Piefed and other Fediverse stuff, XMPP, etc., is themed around the moons of the Solar System! My Piefed account is scallisto (after Callisto, the outermost Galilean moon), but my Vernissage (cool Pixelfed basically) one is seuropa (after Europa, an icy world with a subsurface ocean). For GOG, my username is sbetelgeuse, not named after a moon but an awesome giant star!
For a lot of my non-social accounts, I instead name them after various birds or adjacent things. For instance, my itch.io account is spenguin, my Lichess account is srook_asinthebird (a pun on the chess piece), and speckypeckpeck for Steam (because speck was already taken)
I also have a few oddballs like scloud for my Nextcloud instance, and as well as sgcbird for RetroAchievements (gc meaning GameCube! sgbbird, spsbird, sgbabird, ssnesbird, swiibird, etc. do not work nearly as well)
A few of my accounts are just "sbird", mostly the general ones that are like my "core" services (email, Codeberg/Forgejo, Firefox Sync, and a few others)
a bit of nerding out about the Galilean moons mentioned
Callisto also has the oldest surface in the Solar System dating perhaps 4.5 billion years ago! Like Europa, it has a subsurface ocean (though less likely to contain life), and it could also be a good base for human exploration of the Jovian system, given its low levels of radiation, weak surface gravity, and minimal geological activity!
Europa is probably the most famous moon outside our own, the only outer Solar System moon whose Wikipedia is the top result when you search it by name (at least on Ecosia and Qwant). Europa could potentially contain microbial life in its salty subsurface ocean!